1. Field of the Invention
The present invention relates to an ink jet print head having ejection ports through which ink is ejected, and a printing method and apparatus using the ink jet print head.
2. Description of the Related Art
For a printing apparatus forming images on printing media, particularly an ink jet printing apparatus, an important subject is to increase the speed of color printing on plain papers or the like and to improve the quality of images.
For the ink jet printing apparatus, common techniques for increasing the print speed include, besides an increase in the length of an ink jet print head, an increase in the printing (or driving) frequency of the print head and the use of bidirectional printing. Compared to unidirectional printing, the bidirectional printing temporally disperses energy required to achieve the same throughput. The bidirectional printing is thus effective means in terms of costs of total system.
In the bidirectional printing method, depending on the configuration of the ink jet print head, the order in which each of color inks are ejected may vary between the forward movement and backward movement of the print head in a main scanning direction. This may result in uneven color density shaped like bands. To solve this fundamental problem, for example, Japanese Patent Laid-Open No. 2001-171119 proposes an ink jet print head in which rows of ejection ports (also referred to as “nozzles”) are symmetrically arranged in the main scanning direction.
In the ink jet printing apparatus, a carriage holding the ink jet print head is moved at a high speed in the main scanning direction. In connection with this, if a print image with a high duty, for example, a solid image, is printed, ink droplets ejected through the ejection ports positioned at the respective opposite ends of the ejection port rows in the print head are drawn toward the center of the ejection port rows. Such a phenomenon is also called end deviation and known to be caused as follows.
That is, when ink droplets are ejected toward a print medium through the ejection ports in the ink jet print head, air present around the ink droplets moves in conjunction with the motion of the ink droplets. Thus, the atmospheric pressure in the vicinity of the ejection port rows formed in the ink jet print head tends to decrease compared to that surrounding the ink jet print head. As a result, the surrounding air flows into a decreased pressure area in which the ink droplets are ejected, thereby generating an air flow (such an air flow is hereinafter referred to as a “self air flow”).
The self air flow generated in the vicinity of the ejection port rows is caused by the speed at which the print head moves in the main scanning direction (carriage scanning speed), the print density (print duty) in a predetermined area such as one print scanning area, the distance (head-paper distance) between the print medium and the print head, and the like. That is, the generation of the self air flow relates significantly to printing conditions for the ejection port rows in the ink jet print head.
When a serial scan ink jet printing apparatus is used to form an image while repeating a main scan and a sub-scan under printing conditions that may cause the self air flow, the following problem may occur. A stripe-like uneven density (white stripe) to which no ink is applied may be formed in the joining portion between print scan areas.
Japanese Patent Laid-Open No. 2003-145775 is proposed to reduce possible such white stripes. In this patent document, the arrangement interval (nozzle pitch) of ejection ports included in ejection port groups positioned at the respective opposite ends in the arrangement direction of print elements is set to be larger than that of ejection ports included in an ejection port group positioned in a central area in the arrangement direction. This serves to reduce possible white stripes.
Recently, in the field of ink jet printing apparatuses, there has been a demand to output images of high quality such as a photograph. In order to meet this demand, the ink jet print head tends to provide smaller droplets and have more densely arranged ejection ports as well as a larger length. Printing apparatuses in which such an ink jet print head is mounted tend to allow the print head to perform scanning at a higher speed and to drive at a higher frequency.
In this situation, the occurrence degree of white stripes has been increasing; the white stripes are associated with the end deviation that may occur during high-duty printing such as one-pass printing. Thus, measures for alleviating such an undesirable condition are required. In particular, black ink or the like which has a lower lightness may involve more noticeable uneven density (while stripes) than that of color ink, resulting in a more serious problem.
Thus, an object of the present invention is to use a method totally different from the conventional one to solve the problem resulting from the end deviation in the case of ink with a lower lightness as described above and provide an ink jet print head capable of forming images of high quality at a high speed. Another object of the present invention is to provide a printing method and apparatus using the ink jet print head.